Thread: Slow speed grinders - Vicmarc vs Sherwood

Get the Vicmark. The extra hp is needed to push the heavy CBN wheels.

Regards from Perth

Derek

I hadn't even considered that, cheers. Looks like the Vicmarc it is.

but if you're using a CBN wheel, do you need a slow speed grinder?

There is a slight difference in speeds for vitrified and metal wheels but CBN wheels are optimised to removed stock between 10 and 30 m/s.
On an 8" wheel this translates to ~950 to 2800 rpm with stock removal being related to the speed and pressure used.

CBN wheels can be used outside this range but it won't be as efficient.
At the slower end of the scale expect slow stock removal, so what happens is users end up pushing harder so they think they need a more powerful grinder but all this does is increase the chances of CBN grain loss.

At higher speeds, especially on grinders with a bit of vibe, there is an increase chance of chattering which also increases the chance of knocking CBN grains off the wheel.

If the optimised speeds are used, higher power is only needed for shaping. For sharpening, especially light touchups you don't need a lot of power.
A lower power grinder can be used efficiently for shaping using a lower grit at a higher speed.

This is where a variable speed dual wheel (high and low grit) grinder comes in really handy.
Low grit - high speeds for efficient stock removal and lower speeds - higher grit for light pressure sharpening touchups.

One way to get a variable speed grinder is to get a 3Phase grinder and put a VFD on it.
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Hi Bob

thanks for that detail, but the essence of my question is about the necessity to purchase a slow speed grinder to use with CBN wheels, especially if the slow speed grinder will cost more than the "equivalently powered" high speed grinder.

variable speed via a VFD and 3 ph motor would be nice, but if I'm planning on using a CBN wheel and just want to walk into a tool store and walk out with a bench grinder, can I be guided by price when choosing between slow and regular speed grinders -- assuming that arbor float and similar is the same for the alternatives.

I thought I answered the question but maybe I don't need to be so long winded.

The 1/2HP timbecon is definitely on the light side especially as the 1/2HP is almost certainly INPUT power so maybe 1/3rd OUTPUT power.
The 2HP Vicmarc is perhaps a bit of overkill but you'll end up paying ~$230 for a 1HP 2880 rpm Grinder anyway, although IMHO a 2880 grinder is probably a better all round CBN grinder.

Originally Posted by bueller

I hadn't even considered that, cheers. Looks like the Vicmarc it is.

Which tool rest do you plan to use? And what size/style wheel?

My thought is that there are two types of set ups, which will influence the shape of the wheel. Regardless, the wheel face should be flat and not rounded at the edges - the rounded wheels are for turners and simply lose area for grinding planes and bench chisels.


For wide wheels, such as 1 1/2” (which I use), the advantage is that the face is flat, and never needs dressing. It retains its shape. This means that a tool rest can be set up and retain it settings. I use a Tormek tool rest, and for me it is similar to using a Tormek (except that the wheel is dry and grinds quickly). The Veritas grinder holder (which I used years ago) could be used similarly (but sets up without modification on a 1" wide wheel). Others who use a more traditional tool rest and freehand a blade, that is, no jig (which I do as well) will also enjoy the wide wheel, especially when grinding chisels, since they can be presented straight on to the wheel. I am happy with the wide wheels and would not wish to change for something narrower.


If choosing a narrow wheel, some may prefer the type of set up that Tools For Working Wood sell, that is, a wheel with a cambered face. If you use a cambered face, then wheel width is largely irrelevant. It may as well be narrow to save costs. I used a set up like this when the wheels were white Nortons. It does work very well. This narrow cambered wheel is just a different strategy. It is easier for some to control a grind by working with a small section of the blade. The older friable wheels went out of shape quite rapidly, and the advantage of this style was that is reduced the problem of this failing. My thought is that this is no longer the way that some will want to go as the CBN wheel does not suffer from the same issue.

There are cheap CBN wheels on the horizon. Woodturners Wonders are now selling one for $99 USD that has a composite body.

I still have the 180 grit 8" radiused wheel that started my voyage (replaced with a straight edged wheel), which has had little use, and will sell it for a reasonable sum to anyone interested.

Regards from Perth

Derek

Already bought a Tormek bench grinder kit and will be buying an 8" 40mm wide Vicmarc CBN wheel in 180 grit. Sound good?

Excellent! Sounds my set up!

Regards from Perth

Derek

Originally Posted by ian

but if you're using a CBN wheel, do you need a slow speed grinder?

My experience has been; not for HSS.

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Originally Posted by NeilS

My experience has been; not for HSS.

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If you're thinking about lathe chisels, then a 350 grit CBN wheel would be ideal for sharpening. It is not about heat reduction (since HSS are impervious to heat), but about the smooth, clean edge you can create with these wheels.

Regards from Perth

Derek